1. Field of the Invention
The present invention relates to data communications in general and more particularly to a method and an apparatus for deskewing wavelength division multiplexed data which is transmitted through a dispersive medium.
2. Prior Art
The proliferation of computers and other data handling devices have created a need for more effective and efficient communications systems for interconnecting the computers and/or other data handling devices. The wavelength division multiplexing (WDM) system has been used to interconnect computers and other data handling devices. The WDM communications systems are attractive in that they can provide parallel data transmission on a single transmission link such as a single wire and/or single optical fiber.
In a conventional optical WDM communications system, the individual bits of a group of bits, e.g., a byte, are applied to different encoders in a set of optical encoders. The encoders modulate each bit or channel at a different optical frequency or wavelength. The modulated optical signals are merged into a single beam which is transmitted through a single fiber to a receiving location. At the receiving location, the single beam is separated into its bit signal components to re-establish the originally transmitted byte of data. Optical filters and/or gratings are used to separate the beam into its component parts.
One known problem in using WDM in an optical fiber system is that the respective bits of a byte of data arrive at different times at the receiver. This phenomenon is referred to as bit skew. A probable reason for bit skew is that an optical fiber is a dispersive medium which tends to delay optical signals at different rates. Thus, optical signals at some frequencies are delayed more than optical signals at other frequencies. Since a byte of WDM data contains signals with different frequencies or wavelengths, the effect of dispersion is to delay some bits in the byte more than others.
For relatively low transmission rate and/or relatively short length of transmission link, the amount of bit skew may fall within acceptable limits. However, for relatively high transmission rate and/or long transmission links, the bit skew may become so large that bits in one byte may at least partially overlap with bits in another byte.
Even for relatively long transmission links, if the exact length of the transmission link is known, the retardation or delay of a bit transmitted at a particular wavelength can be readily calculated. However, the exact length of transmission links in practical commercial communications systems is influenced by several conditions and as such is not normally known. For example, the exact length of a transmission link depends on the manner in which it is installed and will vary from one installation to the next, depending on the installer's technique. Even if the exact length is known initially, it can change due to changes in temperature or it may change if connectors and/or sections of the fiber must be replaced after installation.
Furthermore, it is believed that the bit skew problem is aggravated by drifts in the wavelengths of the lasers which generate the light beams and/or shifts in the group delay curves.
The prior art has proposed several techniques for solving the bit skew problem. Among the proposed techniques, perhaps the best technique is the one set forth in U.S. Pat. No. 4,677,618. In the subject patent, the time delays between two different wavelength (sub-channels) are measured. The measured time delays and known information about the fiber are used in an algorithm which predicts the arrival time of bits in the other sub-channels. The prediction scheme requires that the central wavelengths of the transmitting lasers and the fiber group delay curve must be stable. Any unexpected drift of the laser wavelengths or a shift of the fiber group delay may cause the predicted arrival times to be incorrect. As a result, the sampling time would not be at the bit center and the probability of errors would increase.